What do you think will conserve more energy?

I'm thinking about getting an instant hot water heater. It heats water as you need it and eliminates the need and expense of maintaining a tank-full of hot water. Initially, these are pricey but are supposed to save up to 60% on the heat bill.

My front room has two 4'x 8' picture windows which are double paned. Neither of them opens which I find irritating in warm weather. I love the light but heat is seeping through the glass.

Well...

Windows have an R-value of something like 2

New windows(that don't leak cold in or hot out) could save you more in the long run than the water heater,most water heaters are OK on efficiency but they don't run like a furnace either.

What costs you more each winter,the furnace running or the hot water heaters inefficient heating?

Up to you,maybe someone else will pipe in and give opinion.

KBC

A great deal depends on your climate and the hot water usage in the home. KBC is correct, its your judgement since you have not told us the fuel source for either the furnce or water heater. My Rinnai heater will take me 5 years to pay back my initial investment of $1200 using propane. I think you windows will save as much or more fuel and you will feel more comfortable with the windows.

Our fuel sourcel for both hot water and heat is gas. We use as little hot water (cold water for clothes and once a week quick wash on the dishwasher) as possible and keep the thermostat set at 66 degrees.

The winter temperatures average 45 in our area with short dips to 20 sometimes. Are the huge windows more of a heat drain? I keep one unobstructed for the plants.

Old windows usually aren't thermopane or low E glass so replaceing with high efficiency windows will make a big improvemnet. Also, when you swap out the widows you can seal better than the original install. Window sealants and insulation materials are much, much better than years ago. I'd go for the windows.